Color electrophoretic display

ABSTRACT

The present invention is directed to a color display device wherein each of the display cells is filled with an electrophoretic fluid comprising two types of charged pigment particles dispersed in a colored medium. Multiple colors of high quality may be achieved by the present invention.

This application claims priority to U.S. Provisional Application No.61/492,747, filed Jun. 2, 2011; the content of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a color electrophoretic display utilizing twotypes of charged pigment particles dispersed in a colored solvent orsolvent mixture.

DESCRIPTION OF RELATED ART

In order to achieve a multicolor display, color filters are often used.The most common approach is to add color filters on top of black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. When a red color is desired, the green and blue sub-pixels areturned to the black state so that the only color displayed is red. Whenthe black state is desired, all three sub-pixels are turned to the blackstate. When the white state is desired, the three sub-pixels are turnedto red, green and blue, respectively, and as a result, a white state isseen by the viewer.

A major disadvantage of such a technique is that since each of thesub-pixels has a reflectance of about one third of the desired whitestate, the white state is fairly dim. To compensate this, a fourthsub-pixel may be added which can display only the black and whitestates, so that the white level is doubled at the expense of the red,green or blue color level (where each sub-pixel is only one fourth ofthe area of the pixel).

Brighter colors can be achieved by adding light from the white pixel;but this is achieved at the expense of color gamut to cause the colorsto be very light and unsaturated. A similar result can be achieved byreducing the color saturation of the three sub-pixels. Even with that,the white level is normally substantially less than half of that of ablack and white display, rendering it an unacceptable choice for displaydevices, such as e-readers or displays that need well readableblack-white brightness and contrast.

SUMMARY OF THE INVENTION

The present invention is directed to a display device comprising displaycells, wherein each of said display cells is

-   -   a) sandwiched between a first layer comprising a common        electrode and a second layer comprising a pixel electrode, and    -   b) filled with a display fluid comprising a first type of        pigment particles which are white and a second type of pigment        particles which are red, green or blue and said two types of        pigment particles are oppositely charged and are dispersed in a        colored solvent.

In one embodiment, the white pigment particles are formed from TiO₂.

In one embodiment, at least one type of the pigment particles areencapsulated pigment particles.

In one embodiment, the display fluid further comprises a charge controlagent.

In one embodiment, the colored solvent is a black solvent.

In one embodiment, the black solvent is a clear and colorless solventwith non-charged or slightly charged black particles dispersed therein.

In one embodiment, the non-charged or slightly charged black particlesare substantially transparent.

In one embodiment, the non-charged or slightly charged black particleshave a zeta potential of <20.

In one embodiment, the non-charged or slightly charged black particlesare polymeric and are in the form of a transparent polymeric matrix,with dye molecules embedded in the matrix.

In one embodiment, the colored solvent has a color which iscomplementary to the second type of pigment particles.

In one embodiment, the colored solvent is a clear and colorless solventwith non-charged or slightly charged colored particles dispersedtherein.

In one embodiment, the second layer further comprises at least onein-plane electrode and said device further comprises a white backgroundlayer.

In one embodiment, each display cell defines a sub-pixel and threesub-pixels forms a pixel.

In one embodiment, each display cell defines a sub-pixel and twosub-pixels form a pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 c depict how an electrophoretic display of the presentinvention displays different color states.

FIGS. 2 a-2 c depict an alternative design.

FIGS. 3 a-3 e illustrate the color display application of the presentinvention.

FIGS. 4 a-4-d depict a further alternative design.

FIGS. 5 a-5 e illustrate the color display application utilizing thedesign of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an electrophoretic fluid comprisingtwo types of charged pigment particles dispersed in a colored solvent orsolvent mixture.

First Design:

In the first aspect of the present invention, the display fluidcomprises white charged pigment particles and a second type of chargedpigment particles which may be red, green or blue. The two types ofcharged pigment particles are dispersed in a black solvent.

The white charged pigment particles may be any types of white pigmentparticles, including inorganic, organic or polymeric white particles. Toachieve a high light scattering, pigments of a high refractive index areparticularly useful. Suitable white pigment particles may include TiO₂,BaSO₄ and ZnO, with TiO₂ being the most preferred. The white pigmentparticles may be positively charged or negatively charged.

The colored charged pigment particles may also be inorganic, organic orpolymeric particles formed from a pigment, such as pigment red 254(chemical group diketopyrrolopyrrole), pigment blue 15:6 (chemical groupphthalocyanine), pigment green 36 (chemical group Cu phthalocyanine),pigment yellow 155 (chemical group bisacetoacetarylide), pigment red 122(chemical group quinacridone), pigment blue 15:3 (chemical groupphthalocyanine), pigment black 7 (chemical group carbon black) or thelike.

The two types of charged pigment particles may also be encapsulatedpigment particles.

The two types of charged pigment particles are oppositely charged, andthey may exhibit a native charge, or may be charged explicitly using acharge control agent, or may acquire a charge when dispersed in asolvent.

Suitable charge control agents are well known in the art; they may bepolymeric or non-polymeric in nature or may be ionic or non-ionic.

The ionic surfactants as charge control agent may include (a) theanionic type: alkane carboxylic salts, alkane sulfonic salts, such asAerosol OT, alkyl-aromatic sulfonic salts, such as sodiumdodecylbenzenesulfonate, isopropylamine, alkyl benzene sulfonate,phosphates, phosphoric salts or the like, and (b) the cationic type:fatty amine salts, quaternary ammonium salts, alkyl pyridium salts orthe like.

The non-ionic surfactants as charge control agent may include sorbitanmonoesters, polyethoxylated nonionics, polybutene succinimide, maleicanhydride copolymers, vinylpyridine copolymers, vinylpyrrolidonecopolymer (such as Ganex™ from International Specialty Products),(meth)acrylic acid copolymers, N,N-dimethylaminoethyl (meth)acrylatecopolymers and the like.

Fluorosurfactants are particularly useful as charge controlling agent ina fluorocarbon solvent. These include FC fluorosurfactants such asFC-170C™, FC-171™, FC-176™, FC430™, FC431™ and FC740™ from 3M Companyand Zonyl™ fluorosurfactants such as Zonyl™ FSA, FSE, FSN, FSN-100, FSO,FSO-100, FSD and UR from Dupont.

The solvent, in the context of the present invention, may be a coloredsolvent or solvent mixture or alternatively a clear and colorlesssolvent with non-charged or slightly charged colored particles dispersedtherein.

In the case of a colored solvent or solvent mixture, it preferably has alow viscosity and a dielectric constant in the range of about 2 to about30, preferably about 2 to about 15 for high particle mobility. Examplesof suitable dielectric solvent include hydrocarbons such as isopar,decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil, silicone oil, such as DC200 from Dow Corning, aromatichydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzeneor alkylnaphthalene; halogenated solvents such as perfluorodecalin,perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride,3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene,dichlorononane or pentachlorobenzene; and perfluorinated solvents suchas FC-43, FC-70 and FC-5060 from 3M Company, St. Paul Minn., lowmolecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoroethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del.

A black colorant is added to the solvent to generate the black color.Alternatively, the black color of the solvent may be achieved by amixture of colorants to achieve the appearance of a black color.

As stated, the solvent may also be a clear and colorless solvent withnon-charged or slightly charged colored particles dispersed therein. Thenon-charged or slightly charged colored particles are substantiallytransparent and the color transparency comes from the refractive indexsimilarity between the colored non-charged or slightly charged particlesand the solvent in which the particles are dispersed. The non-charged orslightly charged colored particles may have, for example, a zetapotential of <20, preferably <10, more preferably <5 and most preferably<2.

If the refractive index of the non-charged or slightly charged coloredparticles is not matched to that of the solvent in which the particlesare dispersed, the particles may scatter light in the display fluid. Inorder to eliminate problems associated with the mismatch of therefractive indices, the size of the non-charged or slightly chargedcolored particles is preferably in the nanometer range, more preferablyless than 100 nanometers. Materials for this type of non-charged orslightly charged colored particles may include, but are not limited to,commercially available colorants used in the LCD industry for colorfilter applications, such as Clariant's Hostaperm Red D2B-COF VP 3781(i.e., red 254) which is in the class of diketopyrrolopyrrole, HostapermBlue E3R-COF VP3573 (i.e., blue 15:6) which is in the class ofphthalocyanine, or Hostaperm Violet RL-COF O2 VP3101 (i.e., violet 23)which is in the class of dioxazine.

Alternatively, the non-charged or slightly charged colored particles maybe polymeric and are in the form of a transparent polymeric matrix, withdye molecules embedded (e.g., solubilized or dispersed) in the matrix.Since it is easier to match the refractive indices of a polymer matrixand the surrounding solvent, the size of the non-charged or slightlycharged particles does not need to be tightly controlled. Examples ofthis type of non-charged or slightly charged colored particles mayinclude, but are not limited to, dyed polymeric microparticles suppliedby Merck Chemicals Ltd.; dyed polystyrene particles supplied bySpherotech Inc. or the like. For the colored particles with atransparent polymeric matrix, the dye embedded (soluble or dispersible)therein is much more dilute and adjustable. For example, theconcentration of the red dye in the red particles may be adjusted toallow only the blue or green colors to be absorbed and the red color topass through. With a white background to reflect the red color, the redcolor brightness can be maximized.

FIGS. 1 a-1 c depict an example of how a display cell filled with such adisplay fluid may display three different color states.

As shown in FIG. 1 a, a display cell (10) is sandwiched between a firstlayer (11) comprising a common electrode (11 a) and a second layer (12)comprising a pixel electrode (12 a) and the display cell is filled withan electrophoretic fluid comprising white charged pigment particles (13)and blue charged pigment particles (14), dispersed in a black solvent.

The white and blue pigment particles are oppositely charged. Forexample, if the white pigment particles are positively charged, then theblue pigment particles are negatively charged. Accordingly, the twotypes of charged pigment particles (13 and 14) may move towards thecommon electrode (11 a) or the pixel electrode (12 a), depending on thecharge polarity of the particles and the voltage potential differenceapplied to the common electrode and the pixel electrode.

In this example, the common electrode is on the viewing side.

In FIG. 1 a, when proper voltages are applied to the common electrode(11 a) and the pixel electrode (12 a), the charged blue particles (14)would move to be near or at the common electrode (11 a) and theoppositely charged white pigment particles (13) would move to be near orat the pixel electrode (12 a), causing the blue color to be seen at theviewing side.

In FIG. 1 b, when proper voltages are applied to the common electrode(11 a) and the pixel electrode (12 a), the charged blue particles (14)would move to be near or at the pixel electrode (12 a) and theoppositely charged white pigment particles (13) would move to be near orat the common electrode (11 a), causing the white color to be seen atthe viewing side.

In FIG. 1 c, both the blue (14) and the white particles (13) aredispersed throughout the volume of the black solvent. In this case, theviewer sees the black color state. The voltages applied to the commonand pixel electrodes, after the particles are dispersed, may bepositive, negative or zero as long as they are substantially the same.

Second Design:

FIGS. 2 a-2 c illustrate an alternative design of the present invention.As shown in FIG. 2 a, a display cell (20) is sandwiched between a firstlayer (21) comprising a common electrode (21 a) and a second layer (22)comprising one pixel electrode (22 a).

The display cell (20) is filled with an electrophoretic fluid comprisingtwo types of charged particles, white charged pigment particles (23) andcolored charged pigment particles (24), dispersed in a colored solvent.In FIG. 2, the colored pigment particles are blue.

In practice, the colored pigment particles may be red, green or blue andthe color of the colored charged pigment particles and the color of thesolvent in which the particles are dispersed are complementary. Forexample, if the color of the particles is red, the color of the solventwould be green or blue. Likewise, if the color of the particles is blue,the color of the solvent then may be red or green.

Therefore the term “complementary color” refers to red, green or bluecolor and they are complementary to each other. For example, a red coloris considered complementary to green or blue; a green color isconsidered complementary to red or blue; and a blue color is consideredcomplementary to red or green.

Other features (e.g., charge controlling agents and medium) describedabove for the system of FIG. 1 are also applicable to this design.

Colorants for generating the color of the solvent include a variety ofdyes or pigments which are well-known in the art, for example, they maybe, but are not limited to, azo or phthalocyanine dyes or the like.

The colored solvent may be replaced with a clear and colorless solventwith non-charged or slightly charged colored particles solubilized ordispersed therein, as described above.

The white pigment particles may be positively or negatively charged andthe blue pigment particles are oppositely charged. In this example, thecommon electrode is on the viewing side.

In FIG. 2 a, when proper voltages are applied to the common electrode(21 a) and the pixel electrode (22 a), the charged blue pigmentparticles (24) would move to be near or at the common electrode (21 a),causing the blue color to be seen at the viewing side.

In FIG. 2 b, when proper voltages are applied to the common electrode(21 a) and the pixel electrode (22 a), the charged white particles (23)would move to be near or at the common electrode (21 a), causing thewhite color to be seen at the viewing side.

In FIG. 2 c, both types of pigment particles are dispersed in the greensolvent. In this case, a black color is seen at the viewing side becausethe blue and green colors are complementary colors so that the greenreflected light is absorbed by the blue particles and there are enoughof the blue particles to absorb the light reflected off the whiteparticles.

While in the examples of FIGS. 1 and 2, the color of the charged pigmentparticles and the color of the solvent may be varied, as required by thedisplay application.

The display cells as shown in FIGS. 1 and 2 therefore are ideal for acolor display device wherein each pixel consists of three sub-pixels.

For the design of FIG. 1, one of the display cells (sub-pixel) may befilled with a fluid comprising white charged pigment particles and bluecharged pigment particles dispersed in a black solvent, a second displaycell may be filled with a fluid comprising white charged pigmentparticles and green charged pigment particles dispersed in a blacksolvent and the third display cell may be filled with a fluid comprisingwhite charged pigment particles and red charged pigment particlesdispersed in a black solvent.

FIG. 3 illustrates how multiple colors are displayed with a displaydevice comprising the display fluid of the present invention. Eachdisplay cell represents a sub-pixel and each pixel has three sub-pixels.The three display cells, each representing a sub-pixel, are filled withdisplay fluids as described above.

In FIG. 3 a, when a white pixel is desired, all three sub-pixels areturned to the white color state. In FIG. 3 b, when a black pixel isdesired, all three sub-pixels are turned to the black state. FIG. 3 c,when a red color is desired, one of the sub-pixels is turned to red andthe remaining two sub-pixels are turned to the black state for maximumcolor saturation. Similarly, FIG. 3 d and FIG. 3 e display the green andblue colors respectively. Alternatively, in FIGS. 3 c, 3 d and 3 e, oneof the sub-pixels is driven to the color state while the remaining twosub-pixels are driven to the white state for maximum brightness (at theexpense of the color saturation). Further alternatively, in FIGS. 3 c, 3d and 3 e, one of the sub-pixels is driven to the color state while theremaining two sub-pixels are driven to the black and white statesrespectively. Such a full color display can have the same black andwhite characters of a good black and white display, but also show red,green and blue colors of high quality.

FIG. 3 may also be applicable to the design of FIG. 2. The advantage ofthe scheme in FIG. 2 is that the achievable white state will be higherthan that achievable in the scheme in FIG. 1 because the interstitialabsorption in the white state will be one third of that in FIG. 1. Onthe other hand, in order to achieve the best possible black state, theabsorption of the colored fluid, the depth of the display cell and theparticle concentration will have to be tightly controlled.

In order to achieve better whiteness, a fourth sub-pixel may be added inFIG. 3, which sub-pixel can only display either white or black colorstate.

Third Design:

FIG. 4 illustrates a further alternative design. This design is similarto the design of FIG. 2, except that in the second layer (42), there isat least one in-plane electrode (42 b and 42 c) and the pixel electrode(42 a) is sandwiched between the two in-plane electrodes. There are gapsbetween the electrodes.

In addition, there is a white background layer (45), which may be aboveor underneath the second layer. Alternatively, the second layer mayserve as the background layer.

In the example of FIGS. 4 a-4 d, white and blue charged pigmentparticles are dispersed in a green solvent.

The operations of FIGS. 4 a-4 d are similar to those of FIG. 2 a-2 c,when proper voltages are applied to the common, pixel and in-planeelectrodes.

In FIG. 4 a, the blue charged pigment particles move to be at or nearthe common electrode (41 a), the blue color is seen. In FIG. 4 b, thewhite charged pigment particles move to be at or near the commonelectrode (41 a), the white color is seen. In FIG. 4 c, the white andblue pigment particles are dispersed in the green solvent and as aresult, the black color state is seen.

In this alternative design, there is an additional color state (see FIG.4 d), that is, when proper voltages are applied to the common (41 a),pixel (42 a) and in-plane (42 b and 42 c) electrodes, the white and bluepigment particles move separately to be at or near the in-planeelectrodes. In this case, the green color is seen at the viewing side.

Therefore in this alternative design, each display cell can display fourcolor states, black, white, the color of the colored particles and thecolor of the solvent. The color of the colored pigment particles and thecolor of the solvent are complementary to each other.

The colored solvent in this design may also be replaced with a clear andcolorless solvent with non-charged or slightly charged colored particlesembedded therein, as described above.

In this case, each pixel only needs to have two sub-pixels. FIG. 5 is anexample illustrating this scenario. Each display cell represents asub-pixel. Display cell X in the figure is filled with a fluidcomprising white charged pigment particles and red charged pigmentparticles dispersed in a green solvent and display cell Y is filled witha fluid comprising white charged pigment particles and red chargedpigment particles dispersed in a blue solvent.

When a white pixel is desired, both display cells are turned to thewhite (W) state (see FIG. 5 a) and when a black pixel is desired, bothdisplay cells are turned to the black (K) state (see FIG. 5 b). When ablue pixel is desired, cell X is turned to the black (K) state whilecell Y is turned to the blue (B) state (see FIG. 5 c). When a greenpixel is desired, cell X is turned to the green (G) state while cell Yis turned to the black (K) state (see FIG. 5 d). When a red pixel isdesired, cell X is turned to the red (R) color state while cell Y isturned to the black (K) state (see FIG. 5 e). Alternatively, cell X inFIG. 5 c, cell Y in FIG. 5 d or cell Y in FIG. 5 e may be turned to thewhite state instead of black, providing a brighter but less saturatedcolor.

As stated, FIG. 5 is an example, and alternative combinations are alsopossible to achieve the five color states.

It is also noted that the voltages applied to the electrodes illustratedin the designs of FIGS. 1, 2 and 4 are usually in the form of waveforms.In other words, the images displayed by the display devices of thepresent invention may be achieved by the application of a series ofwaveforms. It should also be noted that the electrode configurations of42 a, 42 b and 42 c in FIG. 4 may be of any size and shape as long asthey can collect both sets of particles to different portions of thedisplay cell.

The display cells referred to in the present application may be of aconventional walled or partition type, a microencapsulated type or amicrocup type. In the microcup type, the electrophoretic display cellsmay be sealed with a top sealing layer. There may also be an adhesivelayer between the electrophoretic display cells and the commonelectrode. The term “display cell” is intended to refer to amicro-container which is individually filled with a display fluid.Examples of “display cell” include, but are not limited to, microcups,microcapsules, micro-channels, other partition-typed display cells andequivalents thereof.

While particular forms of the invention have been illustrated anddescribed, it will be apparent that various modifications can be madewithout departing from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation, materials,compositions, processes, process step or steps, to the objective, spiritand scope of the present invention. All such modifications are intendedto be within the scope of the claims appended hereto.

1. A display device comprising display cells, wherein each of saiddisplay cells is a) sandwiched between a first layer comprising a commonelectrode and a second layer comprising a pixel electrode, and b) filledwith a display fluid comprising a first type of pigment particles whichare white and a second type of pigment particles which are red, green orblue, and said two types of pigment particles are oppositely charged andare dispersed in a colored solvent.
 2. The display device of claim 1,wherein said white pigment particles are formed from TiO₂.
 3. Thedisplay device of claim 1, wherein at least one type of the pigmentparticles are encapsulated pigment particles.
 4. The display device ofclaim 1, wherein said display fluid further comprises a charge controlagent.
 5. The display device of claim 1, wherein said colored solvent isa black solvent.
 6. The display device of claim 5, wherein said blacksolvent is a clear and colorless solvent with non-charged or slightlycharged black particles dispersed therein.
 7. The display device ofclaim 6, wherein said non-charged or slightly charged black particlesare substantially transparent.
 8. The display device of claim 6, whereinthe non-charged or slightly charged black particles have a zetapotential of <20.
 9. The display device of claim 6, wherein thenon-charged or slightly charged black particles are polymeric and are inthe form of a transparent polymeric matrix, with dye molecules embeddedin the matrix.
 10. The display device of claim 5, wherein each displaycell defines a sub-pixel and three sub-pixels forms a pixel.
 11. Thedisplay device of claim 1, wherein the colored solvent has a color whichis complementary to the color of the second type of pigment particles.12. The display device of claim 11, wherein the colored solvent is aclear and colorless solvent with non-charged or slightly charged coloredparticles dispersed therein.
 13. The display device of claim 11, whereineach display cell defines a sub-pixel and three sub-pixels form a pixel.14. The display device of claim 1, wherein said second layer furthercomprises at least one in-plane electrode and said device furthercomprises a white background layer.
 15. The display device of claim 14,wherein the colored solvent has a color which is complementary to thecolor of the second type of pigment particles.
 16. The display device ofclaim 15, wherein the colored solvent is a clear and colorless solventwith non-charged or slightly charged colored particles dispersedtherein.
 17. The display device of claim 14, wherein each display celldefines a sub-pixel and two sub-pixels form a pixel.